The present invention is directed to a combustion-powered setting device for driving fastening elements such as nails, bolts and pins, into a receiving material. Setting devices of this kind can be powered by gaseous or liquid fuels that are burned in a combustion chamber and, in so doing, drive a piston for inserting fastening elements.
Generally, metering a balanced amount of fuel to a corresponding amount of air or oxygen, which is used as an oxidizing agent, for each work cycle constitutes a problem. In particular, the air which is drawn from the environment is subject to variations in pressure and temperature which can unfavorably affect the combustion of the air-fuel mixture when this mixture is too rich or too poor with respect to the fuel.
EP 0 597 241B1 discloses a combustion-powered setting device in which the metering of the fuel from the fuel source to the combustion chamber is carried out through a valve which is excitable by means of a solenoid and which is normally closed. Excitation is carried out electronically by means of a switching circuit which responds to the closing of a switch and which opens the valve for a controllable, predetermined time interval so as to enable the liquid fuel to flow from the fuel source to the combustion chamber. However, this is disadvantageous in that the flow velocity of the fuel decreases when the pressure decreases in the fuel source resulting in a discrepancy between the amount of fuel actually metered and the desired amount of fuel. This lowers the performance of the setting device.
Further, DE 42 43 617 A1 discloses a combustion-powered setting device in which a gas inlet valve is opened mechanically in a work cycle, so that fuel flows from a fuel source into a storage space communicating with the surrounding air. A pressure equilibrium and possibly a temperature equilibrium in relation to the surrounding air can take place through this connection, so that a suitable air-fuel mixture reaches the combustion chamber. The fuel then reaches the combustion chamber at a given time proceeding from this storage space. This is disadvantageous in that a loss of fuel may also occur through the connection with the surrounding air.
DE 40 32 204A1 discloses a setting device in which a piston is arranged in a metering chamber and can press a fuel volume out of the metering chamber into the combustion chamber. The metering stroke of the piston can be adjusted by an adjusting screw which is actuated manually. This is disadvantageous in that the manual adjustment is inconvenient. Further, continued metering of the fuel is carried out via a pressure compensation channel so that a loss of fuel may occur.